They can still reject the proposal. Just because they're built upon Chromium, doesn't mean they need to utilise or retain every feature Google adds to it.
Any browser choosing not to implement this would not be trusted and any website choosing to use this API could therefore reject users from those browsers.
That is semantics that wont help the users. They still need to change browser to access their banks website if their bank is enforcing WEI. There is nothing "broken" in the technical sense, the website and browser will be incompatible with oneanother. The blame is clearly on the bank but what's a single user gonna do if this becomes industry standard for banks?
(I am using banks as an example of a service you cant easily avoid, this would also be true for other important stuff like digitialized government access etc ect)
Forking would be insane because of how much code is in Chromium and how much work goes into maintaining it. The realistic thing to do is to keep doing what they've been doing: maintain a modified branch. WEI would just be one of many changes between Google's version and other vendors'.